Metals are used in integrated circuits for interconnects and ohmic contacts, and aluminum and aluminum alloys remain the metals of choice in very large scale integration (VLSI) fabrications because of their low resistivity and compatibility with other processes. However, both pure and alloyed aluminum are susceptible to hillock formation as a result of stress induced by thermal processing. Hillocks are accumulations or buildups of metal in the film. This phenomenon is particularly troublesome in double-layer metallization (DLM) schemes where hillocks in the first layer can lead to interlayer shorting with the second, subsequent layer. It is widely accepted that strain in compressively stressed thin films is relieved by extrusion of the metal at the surface into columnar cells referred to as hillocks. Hillocks greater than one micron (um) in height have been observed. This stress is generated in aluminum thin films as a result of the mismatch of the thermal expansion coefficients between the metal and the underlying layer. Recently, it has been reported by Y. Kamei, et al. in "Ion Implanted Double Level Metal Process," IEDM Technical Digest, p. 138 (1984) that ion implantation of the metal significantly retards the growth of hillocks. This suppression of hillock growth was attributed to surface disordering resulting from ion-induced radiation damage and not to chemical differences between the metal and implanted species. To eliminate the possibility of chemical effects, Kamei, et al. implanted inert argon (Ar.sup.+) ions which was shown to be equally effective in suppressing hillocks.
However, the applicants found in further investigations into hillock suppression in aluminum metal by ion implantation that in almost all cases, the suppression of hillock growth during subsequent thermal cycles, a thermal or anneal cycle being the point of hillock growth, lasts only through one thermal cycle and is not effective for the suppression of hillock growth during a second and subsequent thermal cycles. Since DLM processes require multiple thermal cycles after the patterning of the first metal film, the ion implantation hillock suppression technique discovered by Kamei, et al. is of limited usefulness. It would be desirable for there to be a hillock suppression technique that would be effective for multiple thermal cycles.